It has long been recognized that it is advantageous to provide a very light coating, known as a size or size coating, of suitable composition on glass fibers which are to be employed as reinforcing elements in resinous articles in order to enhance the mechanical reinforcing properties of the glass fibers when they are embedded in the matrix resin of the article to be reinforced. It has also long been recognized that it is advantageous to apply such size coatings to glass fibers by wetting the fibers with a liquid as soon as practicable after forming, frequently an aqueous liquid sizing composition which, when dried in situ, deposits the desired size as a residue lighly coating the surface of the glass fiber. The application of a suitable sizing composition at an early stage serves also to help protect the fibers from damage during the handling which they subsequently undergo, much of this handling being at high speeds or high stress, such as being gathered into multi-filament strands, collected into suitable packages, heated to dry and/or cure the size coating, shipped to the point of use, depackaged, incorporated into the matrix resin which the fibers are to reinforce and shaped with that resin into the desired articles. The size also tends to act as an adhesive to hold the individual filaments together in multi-filament strand.
Such sizing compositions typically contain a film-forming polymer, a lubricant and a coupling agent, all dispersed or dissolved in a liquid medium which is frequently an aqueous medium. The film-forming polymer is usually selected to be reasonably compatible with the matrix resin or resins in which the glass fibers will be embedded as reinforcement. A broad range of oils, waxes, surfactants, etc. have been employed as lubricants. Among the most frequently used coupling agents are organosilanes. Other components, such as surfactants, anti-static agents, colorants, biocides, anti-foam agents, pH regulators, etc., are also frequently included in such sizing compositions.